The present invention relates to computer-based communication systems that are used to locate people or other objects within a building or a campus. These systems can be used for locating personnel, security, inventory tracking, providing access to secured areas, diaries, environmental management, control of computer systems, and a variety of other purposes.
These systems typically comprise portable transmitters (sometimes named active badges because of their small size and application) and receiving stations (located in various locations) which sense the transmissions from the badges. Mobile carriers, such as people or equipment, carry the individual badges which transmit state information (such as identification codes) to the recipient sensor stations, which receive the state information. The sensor stations either store the information internally, report it to other aspects of a computer-based locating system, or perform some local action depending on the application. The badge transmissions may be generated periodically according to internal timers or may be generated by external stimuli (such as pressing a button on the badge). The sensor stations may be mobile or fixed.
The sensor stations are typically located so as to optimize their ability to determine the location of objects or personnel within the parameters of the communication system. The badges and sensor stations transmit via a wireless communication channel (e.g., infrared or radio frequency). The computer-based locating system uses the dynamically acquired state information to provide timely data on the carriers' locations, for locating, adapting the environment for particular carriers, recording in a central database, paging, and for other purposes where locating and identification are important.
From time to time, based on the relative positions of the badges and the sensor stations, a badge's transmission may be occluded from all sensor stations. That is, none of the sensor stations within the communication system are able to receive the badge transmissions due to an obstruction, the badge leaving the parameters of the system, or due to some other interference. The locating system may recognize this occlusion, for example, by comparing its database of active badges with identification codes of recently received transmissions. However, the individual badges and carriers can be totally unaware of the occlusion. This reduces the quality and timeliness of the information provided by the locating system. This reduces its effectiveness and value both to the carrier and to the users of the locating system, especially in critical applications such as their use in hospitals.